Medullary thyroid carcinoma mimicking an autonomous functioning nodule.

A 42-year-old woman with a medullary thyroid carcinoma (MTC) inside of an autonomously functioning thyroid nodule is reported. The patient was initially diagnosed of a hyperfunctioning autonomous thyroid adenoma but the histopathological and immunohistochemistry study were diagnostic for a MTC. This is the first case of a medullary thyroid carcinoma associated with thyroid hyperfunction which was presented as a hot nodule in the scintigraphy. We propose a systematic performance of a fine needle biopsy in all thyroid nodules because of the possibility of carcinoma associated to functioning nodules could be present.